Officially it was closed because of patronage and hasn't been reopened because it's been standardised for freight only traffic. Unofficially it was closed because some drongo didn't like trains, and hasn't been reopened because PTV can't be arsed figuring out the logistics of running trains up there, even though there's some decent condition locos and carriages that have been displaced the Albury line since the introduction of SG vlocities. Instead the old locos are being sold into freight and the old carriages either scrapped or donated to heritage groups

In practice it obviously would be more complicated than "we want train so put train", but the Mildura line would definitely be one of the easiest to reopen. The biggest issue would just be either building a train depot at Maryborough or converting the Ararat or Ballarat depots to dual gauge. But it wouldn't take very much to reopen